1. Field of the Invention
The present invention relates to an image forming apparatus which has a light source for emitting a plurality of laser beams, forms a latent image on an image carrier with a plurality of laser beams emitted from the light source, and forms, onto a printing medium, an image developed on the image carrier, a control method therefor, and a program.
2. Description of the Related Art
Generally, a conventional electrophotographic image forming apparatus forms an image or electrostatic latent image corresponding to an image signal with a laser beam on a photoconductive drum or photoconductive belt. The image forming apparatus develops the latent image, and transfers the developed image onto a sheet, forming an image.
The electrophotographic image forming apparatus needs to scan the photoconductor simultaneously with a plurality of beams in order to increase the speed and resolution.
It is difficult to integrate edge-emitting semiconductor lasers (LDs: Laser Diodes) generally employed as the light source of an image forming apparatus. The number of beams capable of simultaneous scanning and exposure is small (e.g., four). For this reason, it is becoming popular to use, as the light source of an image forming apparatus, a VCSEL (Vertical Cavity Surface Emitting diode Laser) in which a plurality of light emitting points are two-dimensionally arrayed (see Japanese Patent Laid-Open No. 5-294005). The VCSEL can be easily arrayed. By using the VCSEL as the light source, the image forming apparatus can simultaneously scan and expose the photoconductor with a larger number of beams (multi-beam array).
However, when the image forming apparatus uses a multi-beam array such as the VCSEL, the density becomes nonuniform, and a horizontal streak appears in an output image owing to nonuniform exposure on the photoconductive drum or photoconductive belt.
To form a high-quality image, it is important to control the beam quantity. Generally in an image forming apparatus using a light source (multi-beam light source) for emitting a plurality of beams, the quantity of each beam is measured in a predetermined cycle to control the quantity of each emitted beam such that a measured beam quantity coincides with a predetermined one.
The edge-emitting LD conventionally used as a light source emits a main beam forward for image formation, and a back beam backward at a predetermined ratio to the main beam quantity. The main beam quantity can be controlled based on the back beam quantity by incorporating a PD (Photo Diode) in the package of the edge-emitting LD, and measuring (monitoring) the back beam quantity by the PD.
Since the VCSEL does not emit a back beam, a PD for monitoring the beam quantity needs to be arranged outside the package of the VCSEL. Generally in the image forming apparatus, a half-mirror is inserted in the optical path of a beam emitted from the VCSEL. The half-mirror splits a beam emitted from the VCSEL into a beam (main beam) for forming an image and a monitor beam for measuring the beam quantity. The PD measures the quantity of the split monitor beam, and the main beam quantity is controlled based on the monitor beam quantity (see Japanese Patent Laid-Open No. 8-330661).
It is generally known that an optical member such as a half-mirror changes the reflectance and transmittance in accordance with the deflection direction of an incoming light beam. As for the VCSEL, unlike the edge-emitting LD, the deflection direction with respect to the optical axis is not always constant owing to the structure of the VCSEL. Thus, if a plurality of beams emitted from the VCSEL array is split by the half-mirror, the ratio of a transmitted beam and reflected beam differs between beams owing to variations of the deflection direction. As a result, the ratio of the split main beam and monitor beam changes.
If an image is formed while the main beam quantity differs between beams, the exposure distribution on the photoconductor becomes irregular, resulting in poor image quality such as nonuniform density.
As described above, when the multi-beam array is employed, each laser intensity changes owing to variations in the optical member and developing process. A formed image suffers poor image quality such as nonuniform density.